Frequency-selective signal receiver for communication equipments, more particularly telephone equipments

Abstract

In a frequency-selective signal receiver for receiving at least two multi-frequency group codes and equipped with a voice-frequency trap, a broad-band frequency detector and a narrow-band frequency detector are connected in series parallel relative to the frequency coded input signal whereby the receiver channels of the narrow-band frequency detector provided for each single frequency of interest are activated when a conicidence between signals of two groups occurs. Due to the short transient response time of the broad-band frequency detector, character recognition is possible after a short time. The narrow-band frequency detectors are individually adjustable so that in the signal receiver particularly threatened frequency channels can correspondingly be taken into account.

I claim: 
     
       1. A telephone equipment frequency-selective signal receiver for receiving a multi-frequency group code having means for detecting signals including other than certain desired frequencies, wherein the signals received comprise several simultaneously occurring single frequencies, said received signals being applied to clipping means for conversion to a rectangular waveform, said receiver including first frequency detection means having a broad bandwith frequency detector for each desired frequency of said group and responsive to said simultaneously occuring single frequencies in said rectangular waveform for detecting said single frequencies in said received signal and indicating the occurrence of at least one desired frequency, an output section having an activating gate coupled to the output of said first frequency detecting means and responsive to said indication from said first frequency detecting means, second multi-channel detection means having a narrow bandwidth frequency detector assigned to each of said desired frequencies and responsive to two successive half-cycles of said rectangular waveform representing each of said single frequencies for indicating that at least one of said received simultaneously occurring single frequencies is not a member of said group, said activating gate being responsive to said indication from said second multi-channel detection means to inhibit the response of said output section to said first frequency detecting means, and second gate means responsive to said indication from said first frequency detection means for actuating said second frequency detection means. 
     
     
       2. A frequency selective receiver as claimed in claim 1 wherein said output section is initially responsive to said indication from said first detection means of simultaneous occurrence to allow output of said received signal, and secondarily responsive to said second detection means indicating at least one received signal frequency is not a member of said one group for preventing an output of said received signal. 
     
     
       3. A signal receiver as claimed in claim 1 wherein said second frequency detection means comprises a counter having an area assigned to each frequency of said one group, each said counter area being responsive to each slope change of the rectangular voltage waveform representing the signal frequency assigned to said each area to restart the counting process, each said counting area receiving said rectangular waveform providing an output signal in response to the count in said counting area between successive slope changes in said received rectangular waveform differing from the count representing the assigned frequency of said area, and gate means responsive to said counting area signal and said detected single frequencies of said first detecting means to provide said counting area signal to said output preventing means.   
     
     
       4. A signal receiver as claimed in claim 3 including a monostable multivibrator coupled between said clipping means and said second detection means for providing spike signals indicating said slope changes in said rectangular waveforms to said counting areas for starting and stopping the count in said counting areas. 
     
     
       5. A signal receiver as claimed in 1 wherein said second detection means comprises a plurality of delay circuits, one of said delay circuits being assigned to each frequency of said one group, each of said delay circuits having a delay time corresponding to one-half the period of the assigned single frequency, and coincidence gate means receiving an output from one of said delay lines exceeding said delay time coinciding with a signal representing one of said detected single frequencies from said first detection means for providing said delay line output signal to said output preventing means. 
     
     
       6. A signal receiver as claimed in claim 5 including a monostable multivibrator coupled between said clipping means and said second detection means and responsive to each said slope change of said rectangular waveforms for providing an input signal of appropriate duration to said delay circuits of said second detection means. 
     
     
       7. A receiver as claimed in claim 1 wherein said second gate means are responsive to the output of said first detection means to activate only the detectors appropriate to the frequencies detected by said first frequency detector.